1. Field of the Invention
The present invention relates to an optical cutting method for cutting out a disk-shaped film from a sheet film, and an optical cutter employed therefor.
2. Related Background Art
Optical information-recording-reproducing apparatuses for recording and reproducing information by use of light include ROM type ones allowing reproduction only, WORM type ones (or a DRAW type ones) allowing only one writing operation, erasable type ones allowing repeated writing and erasing, and other types. Any of these types of the apparatuses have characteristics of permitting high-density recording, exchangeability of information recording media, and so forth. Among them, photomagnetic disk apparatuses are highly perfected as an erasable type, and are attracting attention.
In these optical disk apparatuses, a microspot light beam of a size of as small as approximately 1 .mu.m has to be projected onto the information recording face of the optical disk for reproducing and writing information in high density on the disk-shaped optical information recording medium (hereinafter referred to as an "optical disk"). Accordingly, a light-transmissive substrate (usually having a thickness of about 1.2 mm) constituting the optical disk should satisfy severe requirements as to the optical properties, including precision of thickness, uniformity of refractive index, retardation of birefringence, and so forth.
Furthermore, on the light-transmissive substrate, grooves or pre-pits have to be formed for guiding an optical head, which renders the light-transmissive substrate constituting the optical disk expensive. Heretofore, injection-molded polycarbonate resins have been investigated to be used for the light-transmissive substrate having grooves or pre-pits. However, light-transmissive substrates are still expensive because of restrictions on the conditions under which they can be molded and the use of resin components for suppressing the aforementioned birefringence, which makes optical disks expensive and retards the spreading use of optical information-recording-reproducing apparatus.
A method for producing optical disks at a low cost is disclosed in Japanese Patent Publication No. 63-31847. In this method, a film in a form of a wound sheet is employed, and grooves or information pits which have preliminarily been prepared on an original disk are transferred thereon continuously according to a 2P method to produce optical disks at a low cost. In another method for forming grooves or pits on a sheet film, a sheet of a thermoplastic resin is formed by employing a hot roller having an original disk to transfer simultaneously and continuously the grooves or information pits onto the sheet film.
From the sheet film prepared as described above, an individual base film for disk should be cut out with high positional precision without impairing the optical characteristics, especially flatness and low birefringence. However, no apparatus suitable therefor has been found. For example, known stamping machines for stamping out a disk-shaped film by mechanical cutting are not satisfactory in terms of their positional precision. In particular, known stamping machines are unsatisfactory for making optical disks for a peripheral memory device of a computor, which require precision in determining the inside diameter (or concentricity) for inserting a circular metal plate at the center portion of the optical disk.
Moreover, for the purpose of producing optical disks at a high speed and at a low cost, the conventional methods of mechanical cutting cannot simultaneously satisfy the requirements (a), (b), and (c) below:
(a) suitability for a continuous production process, PA0 (b) high precision in the cutting position, and PA0 (c) no deterioration of optical performances required for an optical disk (flatness, low birefringence, etc.).